1. Field of the Invention
The present invention relates to conversion of gases, for example, the conversion of exhaust gases from an internal combustion engine.
2. Discussion of the Background
Various proposals have been made with regard to exhaust gas converting apparatuses that use a honeycomb structure of porous ceramic as the carrier of catalyst that is used for converting the exhaust gas emitted from an internal combustion engine of vehicles such as buses and trucks, construction machines, or the like. Such a carrier of catalyst supports thereon a noble metal catalyst of platinum (Pt), rhodium (Rh), palladium (Pd), or the like, by interposing a catalyst carrier layer of oxide such as γ-Al2O3 (alumina). It has been generally practiced in the art to use γ-alumina for the catalyst carrier layer for increasing the specific surface area, while there are proposals these days to use γ-Al2O3 (alumina) in combination with a material such as ceria (CeO2), zirconia (ZrO2), titania (TiO2), or the like, for facilitating conversion of exhaust gas by using the chemical nature of the carrier.
Because of large adsorptive interaction with platinum supported thereon, it is known, particularly, that ceria can provide the effect of suppressing sintering (grain growth) of platinum. It should be noted that, when there is caused sintering in platinum while the catalyst carrier is being used for the exhaust gas converting apparatus, a decrease in the specific surface area of platinum is caused, and hence a decrease in active sites. This leads to degradation of exhaust gas converting power of the catalyst. On the other hand, ceria has the nature of causing sintering relatively easily, and thus, there are cases in which platinum supported upon ceria undergoes sintering together with the sintering of the ceria.
Japanese Laid-Open Patent Application 2006-43683A describes an exhaust gas converting catalyst having a catalyst carrier in which primary particles of ceria and primary particles of alumina are mixed with each other and platinum is supported upon this catalyst carrier. This catalyst carrier is obtained by a process of forming a mixed sol of a ceria sol and an alumina sol, forming a coagulation product by causing coagulation of the ceria sol and alumina sol from the mixed sol, and drying and firing the obtained coagulation product. The entire contents of Japanese Laid-Open Patent Application 2006-43683A are incorporated herein by reference.